Natural Selection

What do you think term speciation is referring to? Monday, March 2nd What do you think term speciation is referring to?

Natural Selection Process that results in adaptation of a population to the biotic and abiotic environments Biotic: includes organisms that seek resources through competition, predation, and parasitism Abiotic: Includes weather conditions dependent upon temperatures and precipitation

Evolution by natural selection requires: Variation: Members of population differ from one another Inheritance: Many of these differences are heritable genetic differences Differential Adaptedness: Some of these differences affect how well an organism is adapted to its environment Differential Reproduction: Individuals better adapted to the environment are more likely to reproduce

Types of Selection Directional selection Disruptive selection Favors individuals at one end of the phenotypic range Disruptive selection Favors individuals at both extremes of the phenotypic range Stabilizing selection Favors intermediate variants and acts against extreme phenotypes

(a) Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators. (b) Disruptive selection favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage. (c) Stabilizing selection removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against. Phenotypes (fur color) Original population Original population Evolved Frequency of individuals

The Preservation of Genetic Variation Various mechanisms help to preserve genetic variation in a population Diploidy: Maintains genetic variation in the form of hidden recessive alleles Creates a heterozygous advantage